The present invention relates to an optical system for an optical pickup device, a coupling lens used for the aforementioned optical system and an optical information recording/reproducing apparatus which has therein the optical system and conducts at least one of recording and reproducing of optical information.
In the diffusion of optical discs such as CD and DVD, an optical information recording medium capable of recording optical information has become the center of public attention. In the optical information recording medium capable of recording these optical information, for example, in the optical information recording medium utilizing phase changes, energy of the spot of image formation is important. In this case, even when light sources each having the same angle of divergence are used and even when NA of the objective lens closer to the medium is the same, it is clear that establishment of a coupling lens having the positive refractive power on the light source side of the objective lens shown in FIG. 3 is more advantageous on the point of the light source energy efficiency, compared with an occasion where no coupling lens is provided. In this case, when the optical system magnification for the forward direction is different from that for the return direction as shown in FIG. 3, the following points are feared.
Namely, an object point (light source light-emitting position) and an image point (optical information recording medium image formation position) both in the forward optical system are fixed, and an object point (optical information recording medium position) and a sensor position of a photo-detector both in the return optical system are fixed. If a decline of a refractive index in the course of temperature rise is caused, it is necessary to drive the objective lens in the direction for the objective lens to part from the optical information recording medium, when excessive spherical aberration is caused on the forward optical system and when trying to form a spot satisfactorily on the optical information recording medium. In this case, however, following troubles are caused.
If the objective lens is driven by an actuator in a way to obtain the best image point on the forward optical system, an excellent image formation spot is formed on at least on optical information recording medium. However, an image formation magnification is changed on the forward optical system because the objective lens is driven. In this case, with respect to the mechanical light-emitting point and the image formation point of the return optical system, the distance between them is constant in the same way as in the distance between a disc and a sensor. Therefore, the distance between the optical image formation point and a mechanical image formation point becomes great, causing harmful influences.
One of the harmful influences is that recording or reproducing is conducted as an optical pickup device under the condition which is not optically excellent image formation, because the mechanical image formation point is given priority. Another harmful influence is an occurrence of a trouble that a numerical aperture of the objective lens on the light emergence side is also changed when the image formation magnification on the forward optical system is changed, and a desired spot diameter cannot be obtained on the surface of an optical information recording medium.